1. Field of the Invention
This invention relates to electro-optic color display systems. More particularly, it relates to electro-optic color display systems with decoders that implement bi-directional row scanning and pre-writing.
2. Discussion of the Related Art
Display systems having colored light bars that sequentially scroll across an electro-optic light panel to produce a color image are well known. Such display systems are particularly useful for displaying color images that are continuously updated by frames, such as in color televisions. Typically, each frame is composed of color sub-frames, usually red, green and blue sub-frames.
Such display systems employ an electro-optic light panel that is comprised of individual pixel elements that are organized in a matrix of rows and columns. The individual pixels elements are modulated in accordance with pixel image information. Typically, the pixel image information is applied to the individual pixel elements by rows during each frame period. Such a matrix array of pixel elements is preferably xe2x80x9cactivexe2x80x9d in that each pixel element is connected to an active switching element of a matrix array of switching elements.
Because each color sub-frame must be addressed during each frame period, the sub-frame addressing rate is three times faster than the frame rate. At present, a preferred electro-optic light panel is a reflective active-matrix liquid crystal display (AMLCD) that is produced on a silicon substrate and that employs a twisted nematic (TN) effect liquid crystal. Thin film transistors (TFTs) are usually used as the active switching elements. Such panels can support a high pixel density because the TFTs and their interconnections can be integrated onto the silicon substrate. Moreover, reflective active-matrix liquid crystal displays can be addressed at a much higher rate than transmissive active-matrix liquid crystal displays. However, a TN reflective active-matrix liquid crystal display requires about 100 microseconds to image a pixel element. In contrast, a row of pixel image information can be produced and applied to the pixel elements in about 5 microseconds. Another problem with current reflective TN active-matrix liquid crystal displays is that the pixel capacitance varies according to the applied voltage.
One problem with taking a relatively long time to image a pixel element is that the image accuracy of the pixel depends on that pixel""s residual state, which in turn depends on previously imaged information. This means that the brightness of a particular pixel depends on the brightness of the previous image displayed by that pixel. Two-dimensional look-up tables can be used to provide correction values for new pixel image to correct for residual states.
The problems of slow response time and varying pixel capacitance versus voltage in reflective TN active-matrix liquid crystal displays can be reduced by using an electro-optic material having a faster response time and a reduced voltage-dependent capacitance. One class of such materials is the ferroelectric LC. However, ferroelectric LC materials have a memory effect in that the image that was produced (the prior image) must be overcome by a new image. Auxiliary xe2x80x9cblanking pulsesxe2x80x9d that reset the pixels prior to imaging new pixels can significantly reduce the memory effect problem. Such blanking pulses can be applied during a line selection period via row electrodes in combination with a common counter-electrode. In practice, the use of two xe2x80x9cpre-writexe2x80x9d blanking pulses has proven more successful than using a single xe2x80x9cpre-writexe2x80x9d blanking pulse.
Pre-write blanking schemes usually require special circuitry for generating the blanking pulses. In the prior art, that special circuitry was not readily integrated into the driver circuitry that converted incoming pixel information, which is usually digital, into analog signals suitable for driving the active-matrix liquid crystal display.
Prior art circuitry for driving active-matrix liquid crystal displays usually used shift registers. However, in scrolling color applications (such as with a computer display screen), non-contiguous rows sometimes need to be accessed. Thus, multiple shift registers, operating in parallel, are required. Furthermore, if bi-directional scanning is desired, even more dedicated shift registers are required.
A known alternative to shift registers in some applications is the decoder. Decoders can enable random row selections. However, prior attempts to use decoders for presenting row information, producing pre-writes to compensate for memory effects, and to implement bi-directional scrolling proved impractical. Therefore, a new technique of using decoders to address rows (or columns) of a display device would be useful. Even more beneficial would be a new technique of using decoders to implement random row (or column) selection, pre-writes, and bi-directional scrolling of display devices.
The principles of the present invention provide a new technique of using decoders to implement random row (or column) selection and pre-writes in a display. Those principles can further enable bi-directional scrolling.
Drive circuitry according to the principles of the present invention can operate an electro-optic display device such that color artifacts caused by residual states are reduced or eliminated by pre-write blanking pulses. That drive circuitry can also implement bi-directional scrolling. Such drive circuitry includes a plurality of decoders, each connected to an address bus, each having a row select enable, and each producing a row select signal for a row of a pixel array. Select signals from the various decoders are combined for each pixel in the pixel array row together to produce pixel drive information for a pixel driver. Beneficially, each decoder is connected to the same address bus, and each row select enable signal is produced by a common controller. By using the row select enable lines, in synchronization with address information on the address bus, the correct pre-writes and image information is applied to a pixel driver for each row of pixels.
In accordance with the principles of the present invention, color artifacts caused by the residual states of the pixels in an electro-optic display device from previously addressed data signals are substantially reduced or eliminated by signals from at least one of the plurality of decoders, while image information is produced by another of the plurality of decoders.
Preferably, the common controller enables the decoders, as required, to produce a desired image, to pre-write row of pixels to prepare for the next image, and to enable bi-directional scanning.